 #1
 14
 0
Hi, I am also having trouble with the hockey puck question.
A hockey puck rebounds from a board as shown in my diagram. The puck is in contact with the board for 2.5 ms. Determine avg acceleration of the puck over the interval.
V_{i} = 26 m/s V_{f }= 21 m/s
I tried the cosine law but I keep getting 44 m/s and not 18. I don't understand how you guys got 18 m/s. I've plugged it in at least 100 times as
[itex]
v_t = \sqrt{v_1^2 + v_2^2  2(v_1)(v_2)cos136}
= \sqrt{26^2 + 21^2  2(26)(21)cos136}
=44
= [/itex]
Because that wasn't working I then tried Vector Components and I can't get that to work either. I did:
[itex]
V_x = V_B sin \theta + (V_A cos \beta )
= 21 sin(22)  26 cos(22)
=16
[/itex]
and
[itex]
V_y = V_B cos \theta + (V_A sin \beta )
= 21 (cos22) + 26(sin22)
= 29
[/itex]
I then tried to figure out
[itex]
\Delta V ^2= \Delta V_x ^2 + \Delta V_y^2
= sqrt{16^2 + 29^2}
= 33
[/itex]
Using that I tried to get the average acceleration by:
[itex]
A_av = \Delta V / \Delta T
A_av = 33 / 2.5x10^3
A_av = 13.2x10^3
[/itex]
and to find the angle I tried to do :
[itex]
\phi = tan^1 = 16/29
\phi = 29degrees
[/itex]
However, the answer in my book says that the average acceleration is [itex] 7.3x10^3 [7.5degrees North of West][/itex]
Any help would be amazingly appreciated. Thanks.
A hockey puck rebounds from a board as shown in my diagram. The puck is in contact with the board for 2.5 ms. Determine avg acceleration of the puck over the interval.
V_{i} = 26 m/s V_{f }= 21 m/s
I tried the cosine law but I keep getting 44 m/s and not 18. I don't understand how you guys got 18 m/s. I've plugged it in at least 100 times as
[itex]
v_t = \sqrt{v_1^2 + v_2^2  2(v_1)(v_2)cos136}
= \sqrt{26^2 + 21^2  2(26)(21)cos136}
=44
= [/itex]
Because that wasn't working I then tried Vector Components and I can't get that to work either. I did:
[itex]
V_x = V_B sin \theta + (V_A cos \beta )
= 21 sin(22)  26 cos(22)
=16
[/itex]
and
[itex]
V_y = V_B cos \theta + (V_A sin \beta )
= 21 (cos22) + 26(sin22)
= 29
[/itex]
I then tried to figure out
[itex]
\Delta V ^2= \Delta V_x ^2 + \Delta V_y^2
= sqrt{16^2 + 29^2}
= 33
[/itex]
Using that I tried to get the average acceleration by:
[itex]
A_av = \Delta V / \Delta T
A_av = 33 / 2.5x10^3
A_av = 13.2x10^3
[/itex]
and to find the angle I tried to do :
[itex]
\phi = tan^1 = 16/29
\phi = 29degrees
[/itex]
However, the answer in my book says that the average acceleration is [itex] 7.3x10^3 [7.5degrees North of West][/itex]
Any help would be amazingly appreciated. Thanks.
Attachments

231.8 KB Views: 389